Thermostat
A small valve that controls coolant flow to keep the engine at the right temperature.
In plain English
The engine is cooled by a liquid called coolant, which carries heat to the radiator at the front of the car, where passing air cools it down. The thermostat is a small heat-sensitive valve that sits between the engine and the radiator. When the engine is cold it stays closed so the engine warms up quickly, and once the engine is up to temperature it opens to let coolant flow through the radiator. It has nothing to do with the temperature dial for the cabin, although a faulty one can affect how well the heater works.
Why it's on your estimate
A shop recommends a new thermostat when it sticks open or sticks closed. Stuck open, the engine runs cooler than it should: the temperature gauge stays low, the heater blows lukewarm air, and the check engine light may come on. Stuck closed, coolant cannot reach the radiator and the engine overheats.
How urgent is it?
Fix soon
A thermostat stuck open usually lets you keep driving for a short while, but an engine that runs too cool is less efficient and the fault should be repaired soon. It becomes a do-not-drive problem if the temperature gauge climbs toward hot or an overheating warning comes on, because an overheated engine can be badly damaged.
Questions to ask the shop
- Is the thermostat stuck open or stuck closed, and how did you test it?
- Did you check the coolant level and look for leaks as well?
- Does the housing or gasket need replacing along with it?
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